Foam metal construction and a method for making it



April 25, 1967 DU 5; ET AL 3,316,158

FOAM METAL CONSTRUCTION AND A METHOD FOR MAKING I'l Filed April 1. 1963 Mll/f/VTOEJ OU VALU 6. DUPAEE DONALD E. 5775 wA/er A ro/z/va United States Patent Ofitice 3,316,158 FOAM METAL CONSTRUCTION AND A METHOD FOR MAKING IT Donald G. Du Pree, Tujunga, and Donald E. Stewart, Arcadia, Califi; said Stewart assignor to Electra-Optical Systems, Inc., Pasadena, Calif.

Filed Apr. 1, 1963, Ser. No. 269,514

4 Claims. (Cl. 204-7) The present invention relates in general to the electroplating art and more particularly relates to an electroplating technique by means of which a porous metal construction is produced.

It is oftentimes necessary to to support a thin or otherwise flimsy structure. Merely by way of example, it is well known that thin-walled structures can be produced by electrodeposition of adherent metallic coatings on the surfaces of suitably prepared forms, such as egative molds, matrices, or mandrels. The forms, which can be made from such materials as metal wax, plastic, glass, etc., provide the proper shapes for the metal layers deposited on their surfaces. However once the forms are removed, the structure that remains may warp or otherwise become damaged or destroyed since it is so thin, unless support backing is provided for it.

In doing so, however, several problems are encountered. One such problem is in fabricating a backing structure whose surface configuration Will exactly match that of the thin-walled structure. Otherwise, spaces or gaps will exist between them which may ultimately cause damage to the structure. Fabricating such a perfectly matched backing is obviously a meticulous job and, therefore, time consuming and expensive. Furthermore, where the thin-walled structure is odd shaped, it may be almost impossible to provide a completely satisfactory backing. In addition, providing separate backing poses the further problem on occasion of closely matching the coefficient of thermal expansion of the backing material with that of the thin-walled structure. Where a thin parabolic mirror has been electroformed, for example, it may warp, buckle, or crack if its backing material expands or contracts at a dilferent rate than the mirror itself with changes in temperature.

It is thus seen that there has been a long-felt need for a technique by means of which a more perfect backing could be provided, that is, one that would not introduce the problems or difficulties mentioned and, at the same time, be relatively inexpensive. The present invention eliminates these earlier limitations by making it possible to provide a backing structure as a continuation of the electroforming process used in the manufacture of the abovesaid thin-walled structure. More specifically, in producing the thin-walled structure, Whether it be a mirror or some other device, a mandrel having the desired shape is placed in an electroplating bath where it is made to act as the cathode. By agitating the mandrel, it is freed of gas bubbles that normally would collect around it, thereby ensuring the proper formation of the thin-walled structure on the mandrel. Once the structure is formed, however, to thereafter provide the backing for it, the agitation of the mandrel is halted, thereby intentionally permitting the gas bubbles to collect around the cathode. These bubbles prevent the uniform deposition of the metallic ions and this causes a foamed or porous provide a metal backing layer of the same material to be built up over the facing structure, this further layer constituting the backing.

One of the advantages of such a method is that the backing structure produced by it is bonded to or integral with the facing structure, thereby eliminating one of the aforementioned problems, namely, that of providing a properly-shaped backing structure that will at all places 3,3 l b l 58 Patented Apr. 25, 1967 be in intimate contact with its facing structure. It will also be recognized that the fact that the backing and facing structures are is also an advantage, since this obviously avoids the problem of different thermal expansion rates. Furthermore, because the backing structure is porous in nature, it is lightweight but unusually rigid and strong.

It is, therefore, an object of the present invention to provide a new kind of backing for a thin-walled structure.

It is another object of the present invention to provide a method for producing lightweight but strong metal layers.

It is a further object of the vide a single process for structure and its backing.

It is an additional object of the present invention to provide a metal construction that is light, strong, and has good thermal properties.

The novel features which are believed to be characteristic of the invention, together with further objects and advantages thereof, Will be better understood from the following description considered in connection with the accompanying drawing in which an embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only and is not intended as a definition of the limits of the invention.

FIGURE 1 is a cross-sectional view showing a mandrel over which a mirror structure has been elcctroformed; and

FIGURE 2 is a rear-view of the mirror for the purpose of illustrating the foam or porous nature of its backing, the mirror being partly in cross-section in order to show that the backing is integral with or merely a continuation of the mirror facing.

Consideraing now the invention in detail, a mandrel is first formed having the shape of the desired thin-walled structure. The mandrel may be made of almost any material, such as, for example, glass, metal, plastic, etc. If the mandrel is initially non-conductive because it is made from an insulative material such as glass, it is made conductive by depositing a thin continuous conductive layer over its entire surface, the kind of film deposit used depending upon the conductivity desired, the economics involved, and the desired surface finish. More specifically, materials such as copper, silver, carbon, etc. can be used to provide the conductive film and may be deposited on the mandrel by vacuum deposition, chemical spraying, or by any other suitable method. With the mandrel now conductive, it is mounted on what may be called an agitation assembly which is nothing more than a motor-driven mechanism that will either vibrate, rotate or otherwise move the mandrel in a desired manner. Agitation assemblies of the type mentioned are commercially available and well known and, therefore, it is not deemed necessary here to describe such a mechanism in detail.

Good electrical contact is then made between the mandrel and a low-resistance lead which, in turn, is connected to the negative terminal of a DC. power source, thereby making the mandrel cathodic. Following this, the agitation assembly together with the mandrel mounted on it is then inserted into an electroplating bath, such as a nickel sulfamate bath, the details of which may be obtained from an article by Richard C. Barrett entitled, Nickel Plating From the Sulfam-ate Bath, pages 169- 175, of the Proceedings of the American Electroplaters Society, published in 1954. The intricate details of sulfamate baths may also be obtained from the article entitled, Nickel Plating From Sulfamate Baths, by Myron B. Diggin, published in Metal Progress, in October 1954.

Accordingly, using well known and well established present: invention to proelectroforming a thin-walled now made of the same material lcctrofo-rming techniques, like those described in the bovesaid articles, a layer of metal, such as nickel, is leposited to the desired thickness over the conductive ilm on the mandrel. During this electrodcposition stage, iowever, the mandrel is agitated so as to .get rid of the lydrogen bubbles that collect around it. These bubbles 11'6 large in number and, if not removed, have the effect of shielding the cathode so that no electrodeposit is able to be formed on the mandrel whereat the bubbles are located. As a result, with the bubbles present, the metal skin becomes pitted and, therefore, unsuited to its intended purpose. As already mentioned, the agitation removes the bubbles.

Once the thin-walled structure or metal skin has been formed to the desired thickness, the next step in the process is that of forming the support backing for it. Toward this end, agitation of the mandrel is halted, thereby intentionally initiating the formation and collection of bubbles around the mandrel. As previously mentioned, the bubbles are large in number and prevent a uniform build-up of the metal skin. Consequently, a metal backing for the skin structure is built-up that is of a porous nature, that is to say, of a honeycomb structure. The process is completed when the foam or honeycomb metal b-acking is built-up to the desired thickness, at which time the mandrel is taken out of the bath. Of course, as may be expected, the newly formed structure is then cleaned and removed from the mandrel.

In connection with the formation of the backing structure, it should at this point be mentioned that the pore or honeycomb size of the backing can be controlled by periodically removing the bubbles from around the mandrel. In other words, the honeycomb structure of the backing may be made finer or coarser according to the frequency with which the bubbles are removed from around the mandrel, the higher the frequency the smaller the size of the pores or honeycombs. The bubbles may be removed either by agitation of the mandrel as before, or by a quick or momentary removal of the mandrel from the bath.

Reference is now made to the drawing wherein the fabrication of a mirror is illustrated as an example of the uses to which the above-described method can be put. In FIG 1, a glass mandrel is shown whose upper surface is shaped in accordance with the projected mirror design. Over this upper surface is deposited a thin conductive film 11. In the case of a mirror, silver is used for the conductive film and serves the twofold purpose of making the mandrel electrically conductive and of providing the reflective surface for the mirror The mandrel is then placed in an electroplating bath and, in accordance with electroplating techniques, a metal skin 12, such as nickel, is formed over the conductive film, the mandrel being agitated during this period to get rid of the gas bubbles collecting around it and thereby permit the skin to properly electroform. Next, when the skin is at the desired thickness, the agitation of the rn andrel is stopped but the electroplating process is continued. Consequently, for the reasons previously delineated, namely, the collection of gas bubbles around the mandrel, metal backing 13 that is "built-up over metal skin 12 acquires a foamy or porous or honeycomb structure.

When backing 13 is at the desired thickness, the electroplating process is terminated and the combination is thereafter removed from the bath and cleaned. Mandrel 1G is then removed, leaving the lightweight but strong mirror shown in FIG. 2. As is shown therein, the mirror is made up of film 11 constituting the reflective portion of the mirror, thin-walled structure 12 acting as a base for film '11, and the honeycombed backing structure 13 which rigidly supports the mirror skin. As is clearly shown, the backing structure is merely an inseparable continuation of the skin structure and, therefore, a very firm bond exists between the two. Furthermore, because of the porous or honeycomb attributes of the backing, the entire mirror construction is relatively lightweight.

The mirror illustrated herein and described above is only an example of the many practical applications of the present invention. Accordingly, the invention is not to be limited thereto but is to be considered to include all methods and structures encompassed by the annexed claims.

Having thus described the invention, what is claimed 1. A method of electroplating a honeycoimbed backing structure for the support of a thin-walled str ucture previously electroplated in a desired shape, said method comprising the steps of: establishing a substantially unagitated electroplating bath; immersing a mandrel having the configuration of the thin-walled structure in said "bath and mounting it therein as the cathode thereof; electrodepositing metal onto said mandrel and simultaneously therewith first continuously removing gas bubbles forming around said mandrel cathode until the thinwalled structure is formed thereon and thereafter, until the backing structure is formed, intermittently removing said gas bubbles at a frequency corresponding to the size of the honeycomb porosity desired.

2. The method defined in claim 1 wherein the step of intermittently removing said gas bubbles includes periodically agitating the mandrel immersed in said lunagitated bath.

3. The method defined in claim 1 wherein the step of intermittently removing said gas bubbles includes periodically removing the mandrel from the bath for an interval of time.

4. An electroformed member comprising: a body structure having anterior and posterior portions, said anterior portion being a thin metal skin that has been electroformed in a desired shape and said posterior portion being a honeycombed metal layer that rigidly supports and maintains said metal skin in its shape, said metal layer being integral with and made of the same metal as said metal skin.

References Cited by the Examiner UNITED STATES PATENTS 2,425,022 8/1947 Bart 88105 2,453,668 11/1948 Marisic et a1. 204--49 2,773,561 12/1956 Hunter 183-115 2,879,211 3/1959 Kardos et al. 204-49 FOREIGN PATENTS 15,172 1888 Great Britain. 9,531 1903 Great Britain.

HOWARD S. WILLIAMS, Primary Examiner,

JOHN H. MACK, Examiner.

W. VAN SISE, Assistant Examiner. 

1. A METHOD OF ELECTROPLATING A HONEYCOMBED BACKING STRUCTURE FOR THE SUPPORT OF A THIN-WALLED STRUCTURE PREVIOUSLY ELECTROPLATED IN A DESIRED SHAPE, SAID METHOD COMPRISING THE STEPS OF: ESTABLISHING A SUBSTANTIALLY UNAGITATED ELECTROPLATING BATH; IMMERSING A MANDREL HAVING THE CONFIGURATION OF THE THIN-WALLED STRUCTURE IN SAID BATH AND MOUNTING IT THEREIN AS THE CATHODE THEREOF; ELECTRODEPOSITING METAL ONTO SAID MANDREL AND SIMULTANEOUSLY THEREWITH FIRST CONTINUOUSLY REMOVING GAS BUBBLES FORMING AROUND SAID MANDREL CATHODE UNTIL THE THINWALLED STRUCTURE IS FORMED THEREON AND THEREAFTER, UNTIL THE BACKING STRUCTURE IS FORMED, INTERMITTENTLY REMOVING SAID GAS BUBBLES AT A FREQUENCY CORRESPONDING TO THE SIZE OF THE HONEYCOMB POROSITY DESIRED.
 4. AN ELECTROFORMED MEMBER COMPRISING: A BODY STRUCTURE HAVING ANTERIOR AND POSTERIOR PORTIONS, SAID ANTERIOR PORTION BEING A THIN METAL SKIN THAT HAS BEEN ELECTROFORMED IN A DESIRED SHAPE AND SAID POSTERIOR PORTION BEING A HONEYCOMBED METAL LAYER THAT RIGIDLY SUPPORTS AND MAINTAINS SAID METAL SKIN IN ITS SHAPE, SAID METAL LAYER BEING INTEGRAL WITH AND MADE OF THE SAME METAL AS SAID METAL SKIN. 